1. Technical Field
The present invention relates, in general, to an improved method and system to be utilized with wireless communication systems having cellular architectures. In particular, the present invention relates to an improved method and system, to be utilized with wireless communication systems having cellular architectures, and which dynamically reserve a number of unused channels for the exclusive use of handoffs of calls-in-progress and in sufficient number to service such requests such that blocked calls originating within an individual cell and blocked handoffs of calls-in-progress from other cells are held within acceptable levels. Yet still more particularly, the present invention relates to an improved method and system, to be utilized with wireless communication systems having cellular architectures, and which dynamically reserve a number of unused channels for the exclusive use of handoffs of calls-in-progress and in sufficient number to service such requests such that blocked calls originating within an individual cell and blocked handoffs of calls-in-progress from other cells are held within acceptable levels by creating a dynamic reserve channel pool which is adjusted on the basis of call requests originating inside a cell and handoff requests originating from outside the cell.
2. Description of the Related Art
The present invention is related to wireless communication systems, and, in particular, to wireless communication systems having a cellular architecture (e.g., cellular telephony, Personal Communication Systems, or Global System for Mobil Communication). Wireless communication refers to the fact that transmission between sending and receiving stations occurs via electromagnetic radiation not guided by any hard physical path (e.g., by microwave link.) Cellular architecture refers to the fact that the wireless system effects service over an area by utilizing a system that can be pictographically represented as a cellular grid.
Wireless cellular communication is the latest incarnation of a technology that was originally known as mobile telephone systems. Early mobile telephone system architecture was structured similar to television broadcasting. That is, one very powerful transmitter located at the highest spot in an area would broadcast in a very large radius. If a user were in the useable radius, then that user could broadcast to the base station and communicate by radiotelephone to the base station. However, such systems proved to be very expensive for the users and not very profitable to the communication companies supplying such services. The primary limiting factor of the original mobile telephone systems was that the number of channels available for use was limited due to severe channel-to-channel interference within the area served by the powerful transmitter. Thus, a problem arose as to how to provide more channels within the service area.
Counterintuitively, engineers discovered that channel-to-channel interference effects within the service area were not due solely to the distance between stations communicating with the base transmitter (which intuitively would seem to give rise to the interference,) but were also inversely related to the transmitter power (radius) of the area being served by the transmitter. Engineers found that by reducing the radius of an area by fifty percent, service providers could increase the number of potential customers in an area fourfold. It was found that systems based on areas with a one-kilometer radius would have one hundred times more channels than systems with areas with a ten-kilometers in radius. Speculation led to the conclusion that by reducing the radius of areas to a few hundred meters, the number of calls that could be served by each cell could be greatly increased.
Thus, reducing the power of the central transmitter allowed a significant increase in the number of available channels by reducing channel-to-channel interference within an area. However, as the power of the central transmitter was reduced, the serviceable area was also reduced. Thus, although reducing transmission power increased the number of available channels, the small service area provided by such reduced power did not make such radio telephone systems attractive communication options for many users. Thus, a problem arose relating to how to utilize the discovery that smaller cell sizes increased available channels in a fashion that would provide service attractive to users.
This problem was solved by the invention of the wireless cellular architecture concept. The wireless cellular architecture concept utilizes geographical subunits called "cells" and is buttressed by what is known as a frequency reuse concept. A cell is the basic geographic unit of a cellular system. Cells are base stations (a base station consists of hardware located at the defining location of a cell and includes power sources, interface equipment, radio frequency transmitters and receivers, and antenna systems) transmitting over small geographic areas that are represented as hexagons. Each cell size varies depending on the landscape. The term "cellular" comes from the honeycomb shape of the areas into which a coverage region is divided. Because of constraints imposed by natural terrain and man-made structures, the true shape of cells is not a perfect hexagon, but such shape serves as an effective tool for design engineering.
Within each cell a base station controller talks to many mobile subscriber units at once, using one defined transmit/receive communications channel per mobile subscriber unit. A mobile subscriber unit (a control unit and a transceiver that transmits and receives wireless transmissions to and from a cell site) uses a separate, temporary wireless channel to talk to a cell site. Transmit/receive communication channels use a pair of frequencies for communication--one for transmitting from the cell site base station controller, named the forward link, and one frequency for the cell site to receive calls from the users, named the reverse link. Both the forward and reverse link must have sufficient bandwidth to allow transmission of user data.
The frequency reuse concept is what made wireless cellular communications a viable reality. Wireless communication is regulated by government bodies (e.g., the Federal Communications Commission.) Government bodies dictate what frequencies in the wireless spectrum can be utilized for particular applications. Consequently, there are is a finite set of frequencies available for use with cellular communications. The frequency reuse concept is based on assigning to each cell a group of radio channels used within a small geographic area (cell.) Adjacent cells are assigned a group of channels that is completely different from any neighboring cell. Thus, in the frequency reuse concept there is always a buffer cell between two cells utilizing the same set of frequencies. The cells are sized such that it is not likely that two cells utilizing the same set of frequencies will interfere with each other. Thus, such a scheme allows "frequency reuse" by non-adjacent cells.
Since each contiguous cell utilizes different frequencies, the ability for such a system to supply continuous service across a cell grid requires that a call-in-progress be switched to a new transmit/receive channel as a user transits from one cell into another. That is, since adjacent areas do not use the same wireless channels, a call must either be dropped or transferred from one wireless channel to another when a user crosses the line between adjacent cells. Because dropping the call is unacceptable, the process of "handoff" was created. Handoff occurs when the mobile telephone network automatically transfers a call from wireless channel to wireless channel as a mobile subscriber unit crosses adjacent cells.
Handoff works as follows. During a call, a moving mobile subscriber unit is utilizing one voice channel. When the mobile unit moves out of the coverage area of a given cell site, the reception becomes weak. At this point, the base station controller in use requests a handoff. The system switches the call to another different frequency channel in a new cell without interrupting the call or alerting the user. The call continues as long as the user is talking, and generally the user barely notices the handoff.
The foregoing ideas of cells, frequency reuse, and handoff constituted the invention of the cellular concept. The invention of the cellular concept made the idea of wireless cellular communications a viable commercial reality.
The first large scale wireless communication system utilizing cellular architecture in North America was the Advanced Mobile Phone Service (AMPS) which was released in 1983. AMPS utilizes the 800-MHz to 900-MHz frequency band and the 30 KHz bandwidth for each transmit/receive channel as a fully automated mobile telephone service. Designed for use in cities, AMPS later expanded to rural areas. It maximized the cellular concept of frequency reuse by reducing radio power output. AMPS is utilized throughout the world and is particularly popular in the United States, South America, China, and Australia. AMPS uses frequency modulation (FM) for radio transmission. In the United States, transmission between the mobile and the base station uses separate frequencies on the forward and reverse links.
With the introduction of AMPS, user demand for bandwidth was initially slow until users became acquainted with the power of such a system. However, once users became acquainted with the power of cellular, the demand for the service exploded. Very quickly, even the extended number of channels available utilizing the cellular concepts of reduced power output and frequency reuse were quickly consumed. Users demanded yet more bandwidth, and a problem arose in the cellular industry.
Engineers responded to the problem by devising the Narrowband Analog Mobile Phone Service (NAMPS.) In this second generation of analog cellular systems, NAMPS was designed to solve the problem of low calling capacity. In the NAMPS three transmit/receive channels are frequency division multiplexed into the AMPS 30-kHz single transmit/receive channel bandwidth. Frequency division multiplexing is the process of deriving two or more simultaneous, continuous channels from a propagation medium that connects two points by (a) assigning separate portions of the available frequency spectrum to each of the individual channels, (b) dividing the frequency range into narrow bands, and (c) using each narrow band as a separate channel. Weik, Communications Standard Dictionary 375 (3ed. 1995). NAMPS services three users in one AMPS transmit/receive channel by dividing the 30-kHz AMPS bandwidth into three transmit/receive 10-kHz channels.
Thus, NAMPS essentially tripled the capacity of AMPS. However, although NAMPS tripled the capacity of AMPS, it also introduced significant adjacent channel interference effects. Users did not find such interference acceptable. The problem now was how to maintain the extended capacity of the NAMPS system, but without the interference effects.
This problem was more difficult, because at this point the engineers had pushed the limits of the analog channels of AMPS, via NAMPS, to their absolute data carrying capacity limits. Since the spectrum available to cellular was now being utilized as efficiently as possible, engineers had to find a new way to increase the bandwidth of AMPS, but without the adjacent channel interference introduced by NAMPS. They accomplished this by the overlaying of digital multiplexing technologies onto the analog channels available in AMPS. Such overlaying schemes are generally referred to as Digital AMPS, or DAMPS. North American digital cellular is alternatively referred to as both DAMPS and TDMA. One of the technologies so overlaid is that of Time Division Multiple Access (TDMA.)
Whereas frequency division multiplexing divides a transmit/receive channel into narrow frequency band transmit/receive channels so that more user data can be sent in the original transmit/receive channel, TDMA uses digital techniques to divide time access to an analog channel before users are even allowed to access the analog channel. TDMA uses digital signals and provides each call with time slots into which to insert digital data, so that several calls can occupy one bandwidth. Each caller is assigned a specific time slot. In some cellular systems, digital packets of information are sent during each time slot and reassembled by the receiving equipment into original signal components. TDMA uses the same frequency band and channel allocations as AMPS and NAMPS. Thus, such technology has extended the usable bandwidth of the AMPS to that of NAMPS, but has done so without the adjacent channel interference that is a by product of NAMPS.
Like NAMPS, TDMA provides three channels (i.e. supports three mobile subscriber units) in the same bandwidth as a single AMPS channel (that is, the analog transmission portion of TDMA is very similar to that of NAMPS). Unlike NAMPS, in TDMA digital signal processing is utilized to compress the spectrum necessary to transmit information by compressing idle time and redundancy of messages to be sent over a channel. Once such compressed data has been sent over a channel, sister digital processing equipment on the other end of the channel decompresses the signal. Such compression effectively allows more users to communicate over the bandwidth of AMPS.
AMPS, NAMPS and TDMA are currently being utilized in many parts of the world. AMPS and NAMPS both utilize handoff. Furthermore, since TDMA is digital multiplexing overlaid onto AMPS, TDMA also utilizes handoff.
Thus, AMPS, NAMPS, and TDMA all utilize cellular architecture and some variant of the above described handoff mechanism. For reasons that will now be described, certain facets of the currently utilized methods of effectuating the above described handoff of calls-in-progress from one cell into another cell are deficient.
It was explained above that in order for service to be provided across cells, the frequency reuse concept requires that handoff occurs when a mobile subscriber unit involved in a call-in-progress transits from one cell into another. Inherent within this requirement is that a channel be available within the cell into which the mobile subscriber unit is transiting, where such available channel is used to accept the call-in-progress into the cell.
It was also explained above that user demand for bandwidth in the past has been unrelenting. This demand, rather than subsiding, is growing at the present time. Consequently, if all users within some particular cells are allocated a channel on which to speak, it is quite possible that all the channels such as a cell, will be consumed. If all channels in a cell are consumed by users within that cell, no channels will be available for handoff when a mobile subscriber unit involved in a call-in-progress transits into the cell. Consequently, the call will be "dropped," or experience excessive interference, when it moves out of range of the base station controller of the cell from which it is transiting.
The problem of a cell having no channels available for handoff (and subsequently possibly "dropping" the call-in-progress) is often referred to in the art as "handoff blocking." One method of avoiding "handoff blocking" is to reserve "guard channels" within each individual cells. These reserved channels are then utilized to service handoff requests. Consequently, the ability of such cells to receive a handoff of a call-in-progress from another cell is ensured.
There are deficiencies in the way in which such channels are currently reserved. The deficiencies arise from the way in which guard channels are reserved. Some methods for reserving guard channels are based upon very sophisticated and complex statistical or fuzzy methods which track different variables such as peak and average cell call usage at specific times throughout a day, peak and average duration of call length, peak and average number of handoff requests at specific times throughout a day, etc. These tracked parameters are then numerically processed using high speed digital computers to determine how many guard channels should be reserved for handoff within a cell during different times. The objective of the numerical processing is to utilize the tracked parameters in order to simultaneously attempt to minimize the number of dropped calls due to inadequate reservation of guard channels and to minimize the number of blocked calls (which are blocked due to the fact that guard channels have been reserved) originating within the cell itself. That is, the numerical methods strive to find the optimum number of reserved guard channels at particular points in time.
Since the processing power necessary to implement the numerical processing is not always available, most systems resort to rigidly fixing the number of "guard channels" when such processing power is not available. However, such rigid fixing is not responsive to changing data traffic conditions and often results in either too many blocked handoffs or too many blocked calls.
The presently utilized methods for guard channel reservation generally consume much time and require very high speed computing equipment or result in excessive handoff blocking and/or call blocking. This is generally due to the fact that the present methods for reserving guard channels either track many different variables and subsequently computationally process the variables to determine guard channel reservation parameters, or rigidly fix the number of guard channels. Consequently, such methods either do not operate in real time and are computationally intensive or are non-responsive to changing data traffic. These facts prove disadvantageous and are likely to prove even more disadvantageous as time goes on.
It was mentioned above that as cell size is decreased, the number of channels (and thus users) that can be accommodated within the cell increases. Currently, the industry uses this fact to satisfy increases in user demand for bandwidth. That is, as user demand for bandwidth exceeds the capacity of a cell, the cell is subsequently subdivided into smaller cells having more channel carrying capacity. This operation is known as "cell splitting."
Cell splitting decreases the physical size of the cells. However, assuming mobile subscriber units continue to transit cells at the same velocity as before, it is clear that all equipment in the cells will have to increase in speed in that the decreased physical size increases the speed at which decisions have to be made. For example, a mobile subscriber unit with a call-in-progress traveling eighty miles per hour across 5 cells of approximately 1.0 mile in width will need a handoff approximately (assuming handoff takes place exactly at cell boundaries) every 45 seconds. However, if due to increased user demand for bandwidth each cell is subdivided (split) such that each cell is now 0.5 miles in width, the mobile subscriber unit will need a handoff every 22.5 seconds.
It is apparent that as cell size is reduced and channel density increased a point will be reached where existing guard band reservation methods will be insufficient in that the processing time required by the numerical processing methods will exceed that available to make a decision. Furthermore, rigidly fixing the number of guard channels will be insufficient in that handoff requests are likely to vary greatly dependent upon the number of calls in progress transiting cell boundaries and the velocity at which mobile subscriber units are transiting the cells.
In addition to the foregoing, the current numerical processing methods are deficient in that they tend to be predictive rather than reactive. That is, they tend to use some predefined baseline of historical channel usage and handoff requests to predict future numbers of handoff requests. The number of guard channels are then reserved using these predicted future numbers of handoff requests. Such reservation proves to be deficient if the future numbers of handoff requests vary significantly from the predicted numbers.
Thus, it is apparent that a need exists for a method and system which will perform reservation of guard channels in near real time and such that neither unacceptable amounts of handoff blocking nor call blocking occur within a cell.